POSTIRRADIATION AGING AFFECTS STRESS AND STRAIN IN POLYETHYLENE COMPONENTS

Ultrahigh molecular weight polyethylene components oxidatively degrade because of gamma radiation sterilization and subsequent shelf aging i n air. The effects of shelf aging on the stresses and strains associat ed with surface damage in tibial and acetabular components were examin ed. A material model was developed to predict the stress and strain re lationship of oxidatively degraded polyethylene as a function of densi ty using samples of polyethylene that were gamma radiation sterilized and evaluated immediately after irradiation and after 42 months of she lf aging. The finite element method was used to determine the stresses and strains before and after shelf aging for two tibial components wi th different conformities between the articulating surfaces and for an acetabular component. The stresses increased by 10% to 14% in the con forming tibial model after 42 months of aging, whereas the stresses in the nonconforming tibial model and in the acetabular model increased by only 4% to 8%. Aging decreased the principal strains by 5% to 10% i n both tibial models and by 15% to 17% in the acetabular model. Postir radiation aging during shelf storage of polyethylene joint components is likely to worsen long term wear, based on the increased stresses an d decreased strains predicted to occur as a result of aging.